The present invention relates to a computer system and a motion picture transfer method and, more particularly, to a computer system having a function of decoding and reproducing digitally compressed/encoded motion picture data, and a motion picture transfer method in this system.
In recent years, with the advance in computer technology, various multimedia personal computers have been developed. A personal computer of this type can reproduce not only text data and graphics data but also motion picture data and audio data.
Generally, motion picture data is compressed and encoded in accordance with the MPEG 1, and stored in a CD (Compact Disk). The motion picture data is decoded, displayed, and reproduced using a dedicated expansion board. As an expansion board for decoding, displaying, and reproducing motion picture data, e.g., "REAL Magic" available from Sigma Designs, Inc., USA is well known. This "REAL Magic" has a video decode function complying with the MPEG 1 standard. The decoded motion picture data is synthesized with VGA graphics received from a video card via a feature connector, and the synthesized motion picture is displayed.
The MPEG 1 standard, however, assumes the use of a CD having a data transfer rate of about 1.5 Mbps. Processing motion picture data containing a large amount of image data such as a movie leads to a degradation in image quality, and the like.
Recently, a DVD (Digital Versatile Disk) has been developed as a new-generation storage medium having a data transfer rate about ten times higher than that of the CD. The DVD has a new video disk standard capable of recording video data such as a movie with a high image quality on an optical disk having the same size as that of the CD by using motion picture coding called the MPEG 2. A recording/reproducing method for the DVD is based on variable rate coding in order to ensure both the image quality and the recording time with respect to the capacity. The size of variable-rate encoded data depends on the quality of an original image. A more abruptly changing scene results in a larger data size.
When video data stored in the DVD is to be reproduced on a personal computer, the data must be read from a DVD-ROM in the main memory of the computer, and transferred to a DVD decoder. In this case, if a DVD-ROM interface and the DVD decoder are constituted as a bus master device, the motion picture data can be transferred from the DVD-ROM to the main memory, and from the main memory to the DVD decoder by using the DMA scheme, thereby reducing the load of the CPU.
Even with the DMA scheme, when motion picture data is transferred via the main memory, as described above, the same data flows through the system bus twice (data transfer from the DVD-ROM to the main memory, and data transfer from the main memory to the DVD decoder). Accordingly, the traffic of the system bus increases.
In motion picture data transfer via the main memory, if the contents of a memory area used in the motion picture data transfer are written in another device, the motion picture data read from the DVD-ROM can be illegally copied. Therefore, a copy protect measure to prevent illegal copying must be adopted.